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Auto Select mode on my AC-3 demodulator
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Author:  chileboy [ 29 Sep 2011, 15:58 ]
Post subject:  Auto Select mode on my AC-3 demodulator
I have a B&K DT-1 AC3 RF demodulator which is connected to my Pioneer CLD-97 which has been modded with an AC-3 RF output. In addition to the RF Input connector, it has both Coax-In and TOSLINK-In.

I don't know if it's unique to the B&K unit, since I've never owned any other demodulator, but it will auto-detect whether or not there is an AC-3 signal present; if so, it will use that, otherwise it will pass through the audio from the Coax (or TOSLINK) to the single Coax output. This allows one to use a single input on the preamp or receiver for the LaserDisc player, without having to select one source or the other, depending on the source material (hope all that makes sense).

The issue I have always had with this setup is that when the disc first spins up and the DT-1 "auto-detects", for a split second there is a burst of sound through the system - it seems to just be a random "sample" of the soundtrack whereby, I guess, it does its detective work. It can be quite unnerving when it happens, especially if I'm watching Star Wars or something, and have the sound level set pretty high.

Do other demodulators have this feature, and this problem?

- Mark
Author:  admin [ 29 Sep 2011, 19:05 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
chileboy wrote:
Do other demodulators have this feature, and this problem?


I know of the Pioneer RFD-1 with that capability... but no bursty noise at all!

I wrote a little summary for eBay a long time ago: How to properly handle AC3/Dolby Digital on Laserdisc?

Julien
Author:  chileboy [ 29 Sep 2011, 20:10 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
Julien, thanks for that link - the Pioneer looks suspiciously like my B&K, I'll bet it's the same unit. Now I am thinking, based on what you said, it is the AC-3 mod itself that is causing this! I never suspected that in all this time. I had the mod done by a "laserdisc guy" years ago that I have a lot of faith in - he's repaired and aligned more than one player for me - I am going to try and track him down and ask him about it.

Thanks again.

- Mark
Author:  dgtwoodward [ 30 Sep 2011, 18:27 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
I have a Meridian 519 and it too has the self detect feature, so accordingly has the extra inputs. But it also has Coax and optical outputs too, so you get the choice...depending on which type of input you have free on your amp.

Image

(Ignore the numbers, those I put there to help someone out with a connection question)

I must confess though, It has never made any kind of extra sound or noise except the expected movie audio. It is actually a really fine piece of audio gear, still going for good prices.
Author:  publius [ 30 Sep 2011, 19:47 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
chileboy wrote:
Julien, thanks for that link - the Pioneer looks suspiciously like my B&K, I'll bet it's the same unit. Now I am thinking, based on what you said, it is the AC-3 mod itself that is causing this!

I have heard of this. If the muting (squelch) signal from the audio board is not applied to turn off the AC-3 output during spin-up & certain other operations (eg trick playback) what you describe can result.
Author:  chileboy [ 30 Sep 2011, 20:22 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
publius wrote:
I have heard of this. If the muting (squelch) signal from the audio board is not applied to turn off the AC-3 output during spin-up & certain other operations (eg trick playback) what you describe can result.

Ok, that makes sense - is it fixable? Was something done incorrectly when the mod was done?
Author:  publius [ 30 Sep 2011, 21:14 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
It's not "incorrect", exactly.

The simplest possible AC-3 modification would be a tap on the circuit board leading the AFM audio signal — preferably but not necessarily the 2/R signal after separation — to a jack on the back. This would be a bad idea, because the load on the circuitry could unbalance its operation, & because noise could be led into the player.

The correct method, therefore, is to tap the AFM signal and use it as the input to a "line driver", typically an op-amp. An optional extra step is to tap the "audio mute" signal as well, & use it to switch the line driver on & off in coordination with the regular audio outputs, which is done on "factory" AC-3 equipped players. What you have is probably the simpler of the two modifications.
Author:  chileboy [ 01 Oct 2011, 02:39 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
If I understand what you're saying, I have this mod:

"tap the AFM signal and use it as the input to a "line driver", typically an op-amp."

but without the

"optional extra step to tap the "audio mute" signal as well, & use it to switch the line driver on & off in coordination with the regular audio outputs".

Am I correct? I get that very brief noise burst at the very beginning of each side, but none of the noise or other issues noted in the first, simplest method you describe.

I am also guessing that I can't fix it without different circuitry - I mean, I assume that the op-amp (or whatever) installed needs to have the capability of understanding and accepting the audio mute signal, and I would think if it did, it would have been connected.

Does that all seem correct? Or is the op-amp a pretty standard board used for this modification, and I just need to install the proper connection?
Author:  publius [ 01 Oct 2011, 03:52 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
An op-amp or line driver is one IC, just a little chip. There are a couple of other components which usually go into the mix, a resistor or two & a decoupling capacitor, but basically it depends on the specific type of IC & how it is wired up. Some types would only require a little tinkering to set up to switch on & off according to the squelch signal, while others might be unsuitable, calling for a replacement of the part. It also makes a difference whether the squelch signal on the audio board of your player is easily accessible.
Author:  chileboy [ 01 Oct 2011, 20:00 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
publius wrote:
An op-amp or line driver is one IC, just a little chip. There are a couple of other components which usually go into the mix, a resistor or two & a decoupling capacitor, but basically it depends on the specific type of IC & how it is wired up. Some types would only require a little tinkering to set up to switch on & off according to the squelch signal, while others might be unsuitable, calling for a replacement of the part. It also makes a difference whether the squelch signal on the audio board of your player is easily accessible.

Ouch.

OK, guess I need to contact the fellow who installed it...or, maybe I will have a look inside and see what I find. Sounds a bit beyond my expertise, though.
Author:  laserbite34 [ 16 Oct 2011, 05:28 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
Julien

Read the link and cheers.

Why can’t the output be universally adaptable to work on my Yamaha AVR? Its like trying to get blood out of stone with Dolby AC-3 Laserdiscs and I have some titles I’d like to listen too.

There are some prices for demodulators on ebay often cost more than reasonable priced (buy it now) laserdisc players?
Author:  dizpornbuck70 [ 15 May 2012, 23:32 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
I have similar questions about a recently acquired lexicon demodulator. I've never had or used one before, and am wondering about it's proper hook up and use with my pioneer elite 79 and my onkyo TX-NR609 receiver. Now, what type of cables are needed, connecting the LD to the lexicon, then the lexicon to the receiver (One of the lexicon rear inputs is labeled S/PDIF INPUTS with a coax and optical option, the other RF IN/AC-3, output is labeled S/PDIF)? I'm not quite sure what the difference is between the RF IN AC-3 and the coax S/PDIF input. And, is it supposed to connect to a digital connection in the ONKYo's rear panel? Aside from a series of HDMI options, I have two assignable digital coaxial inputs, one labeled BD/DVD ( The HDMI input with this label has my blu ray player connected to it.) and the other is labeled CBL/SAT (The HDMI input with this label has my cable box connected to it.)
Once hooked up properly, are any other audio cables needed, running from my laserdisc player, or will the demodulator auto detect and apply any AC-3 audio, and allow anything that is not AC-3 to pass through?
Forgive my ignorance, as I am the least technically inclined person I know (Wife aside), and I've found myself sadly stumped with this seemingly simple piece of equipment.
Thanks for any and all advice.
Author:  invenio [ 16 May 2012, 01:59 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
dizpornbuck70 wrote:
I have similar questions about a recently acquired lexicon demodulator. I've never had or used one before, and am wondering about it's proper hook up and use with my pioneer elite 79 and my onkyo TX-NR609 receiver. Now, what type of cables are needed, connecting the LD to the lexicon, then the lexicon to the receiver (One of the lexicon rear inputs is labeled S/PDIF INPUTS with a coax and optical option, the other RF IN/AC-3, output is labeled S/PDIF)? I'm not quite sure what the difference is between the RF IN AC-3 and the coax S/PDIF input. And, is it supposed to connect to a digital connection in the ONKYo's rear panel? Aside from a series of HDMI options, I have two assignable digital coaxial inputs, one labeled BD/DVD ( The HDMI input with this label has my blu ray player connected to it.) and the other is labeled CBL/SAT (The HDMI input with this label has my cable box connected to it.)
Once hooked up properly, are any other audio cables needed, running from my laserdisc player, or will the demodulator auto detect and apply any AC-3 audio, and allow anything that is not AC-3 to pass through?
Forgive my ignorance, as I am the least technically inclined person I know (Wife aside), and I've found myself sadly stumped with this seemingly simple piece of equipment.
Thanks for any and all advice.


You will need standard RCA connector cable for connecting the LD player to your AC3 decoder, and then another one to take it to your Receiver to decode the Dolby Digital stream. There is no pass-through as the only thing coming out of the LD on that connector will be the AC3 stream. If this doesn't solve your issues, try taking a pic of the back of your LD player, AC3 demodulator, and Receiver and I can tell you what to connect where. Hope this helps!
Author:  dizpornbuck70 [ 17 May 2012, 03:04 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
I'm not quite sure how to post photos here of the back of my lexicon, elite 79 and onkyo, but I can say that I have the laserdisc running into the lexicon properly, when an AC-3 encoded disc is played, the lexicon locks the signal. But, where to run the cable into the onkyo is my problem. If I run it with an rca cable into another input that is currently being used by an HDMI cable (My only single cable coaxial choices are DVD and CBL/SAT, both currently in use via their HDMI option), I imagine I would have to switch via on onscreen display for that to work? If that is the case, I would run the audio/video cables into say, the dvd composite input, and the AC-3 into the coaxial digital input labeled DVD. When using one or the other, I would need to manually switch it's assigned input onscreen (Set it to HDMI for bluray, set the composite for laserdisc) As it stands, the regular RCA audio video cables are running into the unused GAME input, and that works fine, but where to run the AC-3 out from the lexicon is confusing. the only options are one of two assignable coaxial digital inputs, one labeled DVD, the other labeled CBL/SAT, which are clearly a separate input, and not possible to run both at the same time. Sorry if I'm being repetitive, but for some reason, this completely baffles me.
Author:  invenio [ 17 May 2012, 13:31 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
It sounds like the hookup to the LD player and AC3 player if fine and that the issue is the hookup into the Onkyo receiver. Is there a picture on the web of the back of your onkyo model? You can post a link. I own an Onkyo TX-SR875 but of course the hookups are different for every model. On my machine the digital coaxial and toslink inputs are independent of the video source designation. This may or may not be the case in your situation. If I can see the back of the player I could probably tell you whether this is the case or not. Another way to post on here is to take a picture, upload the picture to a site like "freeimagehosting.com" and then place the image into the forum message with the picture link (only available on "Full Editor").
Author:  ldfan [ 31 Dec 2016, 09:12 ]
Post subject:  Re: Auto Select mode on my AC-3 demodulator
chileboy wrote:
publius wrote:
I have heard of this. If the muting (squelch) signal from the audio board is not applied to turn off the AC-3 output during spin-up & certain other operations (eg trick playback) what you describe can result.

Ok, that makes sense - is it fixable? Was something done incorrectly when the mod was done?


The squelch noise is indeed common if the mod that was performed on the LD player omitted the "mute" control point.

I had a Sony MDP-455 that I modded years ago that would do that and I could not fix it ever due to the player not having a mute control that was supposedly accessible for the AC3 board that I was using (at least that is what the seller of the board told me). In the end, the player broke down and I eventually moved the board to a Pioneer unit that worked fine with it.


Now.. as it turns out, Invenio wrote up a "how to" on the CLD-97, AC3-RF mod over here.....

Pioneer CLD-97 AC3 mod step by step instructions

He notes that Pin 3 of CN-503 is the mute point so maybe you can open up your player and see if a wire (usually yellow) is soldered to it.
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